Oxidizing compounds, such as peroxygen compounds are widely used in different fields of industries. Perhaps the most commonly known oxidizing compound is sodium percarbonate (SPC), which is a water-soluble crystalline peroxygen compound with the molecular formula 2Na2CO3.3H2O2. Its theoretical active oxygen content (AO) is 15.28% by weight. Sodium percarbonate dissolves relatively fast in water, releasing sodium carbonate and hydrogen peroxide in the solution. Owing to this property, sodium percarbonate has been widely used as bleaching agent. Applications utilising percarbonate are found in textile bleaching and removal of colored stains from textiles in industry and households in particular. In addition, percarbonate is used in a variety of other applications, including fish farming, soil remediation and oil well stimulation.
Sodium percarbonate involves the problem that, although sodium percarbonate as such is a stable compound, the slight amounts of moisture which are present already in the atmosphere and/or washing agents are sufficient to bring about the decomposition of the percarbonate. As a result, it loses its active oxygen. Especially when stored under warm and humid conditions, sodium percarbonate will decompose alone. To improve the storage stability and to increase safety during transport, percarbonate is normally coated with a thin layer of sodium sulphate (5-10% by weight) with or without the inclusion of boric acid, silicate, carbonates or combinations thereof. Such a coating, when exposed to aquatic media, will dissolve within minutes and release the percarbonate.
Thus, the decomposition of sodium percarbonate can be prevented by coating the sodium percarbonate particles.
U.S. Pat. No. 3,951,838 specifically describes a method for preparing sodium percarbonate by contacting particulate sodium percarbonate with an aqueous sol containing 3 to 8 percent W/V of silica in order to deposit on the percarbonate particles from 1 to 10 percent by weight of silica based on the weight of the percarbonate. The silica sol is obtained by decationizing aqueous solutions of water soluble silicate. This patent does not disclose coating in a fluidized bed let alone with an alkali silicate.
GB Patent 1 538 893 discloses a process for stabilizing a particulate alkali metal persalt, such as sodium percarbonate, comprising coating the persalt with a solid coating agent containing sodium carbonate, sodium sulphate and sodium silicate. This patent specification includes comparison examples where sodium percarbonate particles were coated with a sodium silicate solution to yield a coating in an amount of 2.5 or 5% by weight of the uncoated particles. However, such a coating was not satisfactory.
US 2006/0063693 A1 discloses coated sodium percarbonate particles having a delayed release of active oxygen into an aqueous phase, said particles comprising a sodium percarbonate core, an innermost shell layer comprising an inorganic salt and an outer shell layer comprising an alkali metal silicate which has been prepared by using an aqueous solution containing 2-20% by weight alkali metal silicate. The drawback of such a product is that it is complex comprising a number of various components, and additionally the dilute silicate solution brings along much water which has to be evaporated in the process.
EP 0 992 575 A1 discloses alkali metal percarbonate particles having alkali metal silicate uniformly mixed within the particles and having a coating containing alkali metal silicate. In a granulation step alkali metal silicate is added and mixed uniformly with alkali metal carbonate and stabilizers to obtain particles of a suitable size. The granulation may be performed by compacting, extruding, agglomeration in drum or disk, fluid-bed granulation, prilling or in different kinds of mixers. The coating may be performed by spraying in a drum or fluid bed.
EP 0 789 748 B1 discloses particles having a sodium percarbonate core coated with a silicate and a water soluble magnesium salt, the core and/or coating additionally containing a chelating agent. A preferred coating comprises a first layer, counted from the core, containing the silicate, while a second layer contains the magnesium salt. A preferred magnesium salt is magnesium sulfate.
The silicate is preferably sodium or potassium silicate or a mixture thereof and the amount of the silicate in the coating is preferably from 0.05 to 7% by weight SiO2.
EP 1 149 800 B1 relates to coated granular sodium percarbonate having a multi-coated surface with both inner layer and outer layer, wherein said inner layer comprises a mixture of alkali metal silicate and at least one compound selected from sulfate, carbonate, and bicarbonate of alkali metals and said outer layer comprising a mixture of alkali metal sulfate and at least one compound selected from carbonate and bicarbonate of alkali metals. Coated granular sodium percarbonate containing magnesium salts are disclaimed from this patent. Clearly, this patent discloses complex structures wherein the inner layer comprises alkali metal silicate+another defined component and the outer layer comprises alkali metal sulfate+another defined component.
EP 0 623 553 B1 discloses a stabilized sodium percarbonate particle comprising a sodium percarbonate particle having at least one coating layer thereon, wherein the at least one coating layer comprises: a silicate; magnesium sulfate; and an alkali metal salt selected from the group consisting of alkali metal carbonates, alkali metal bicarbonates and alkali metal sulfates.
The specification describes structures wherein the three components are distributed within two layers, for example in such a matter that the silicate is in the first layer and the magnesium sulfate and alkali metal sulfate in the second layer. Due to the great number of components also these structures are complex.
WO 00/57022 discloses a method for treating an underground reservoir by introducing into the reservoir a treatment fluid comprising an ester and a polymer breaker, such that the ester hydrolyses to produce an organic acid to dissolve acid soluble material present within the reservoir and the polymer breaker degrades polymeric material present in the reservoir. The polymer breaker may be an enzyme or an oxidant, such as persulphate, hypochlorite, peroxide, perborate, percarbonate, perphosphate or persilicate.
U.S. Pat. No. 5,054,552 relates to breaking of high viscosity fluids containing xanthan gums. The disclosed breaker comprises a combination of e.g. sodium percarbonate and ammonium persulfate.
U.S. Pat. No. 4,552,674 discloses an aqueous composition suitable for treating a subterranean formation comprising a hydratable polymer, a peroxygen compound and an activator. The peroxygen compound includes e.g. sodium perborate, sodium percarbonate or hydrogen peroxide.
GB 2 426 973 A discloses a method for removing odours from effluent comprising contacting dewatered effluent with sodium percarbonate granules, wherein the granules are coated with a compound that is sparingly soluble in water. The sodium percarbonate may coated with at least one compound including magnesium silicate, magnesium carbonate, magnesium sulphate, sodium silicate, sodium carbonate, sodium hydrogen carbonate or aluminium silicate. The coating amounts to 0.5 to 25 weight % of coating compound relative to sodium percarbonate. This document teaches that if the number of coating layers is one, the coating amounts to up to 1.5 weight % relative to sodium percarbonate.
JP 11158016 discloses a tablet intended to be placed in the exhaust port of a kitchen sink etc. for preventing formation of slime and having a deodorizing effect. The tablet comprises sodium percarbonate coated with boric acid and/or an inorganic salt, for example with boric acid and water glass, and an organic powder and inorganic powder.
In one aspect, the present invention generally relates to well treatment fluid compositions and methods of use, and more particularly, to well treatment fluids and methods that include a delayed release peroxygen compound formulation.
The internal pressure in an oil well forces only about the first 3 percent to the surface and 10-20% can be acquired by traditional pumping. Gaining access to at least part of the remaining oil requires more advanced technology. In order to gain access, viscous well treatment fluids are commonly used in the drilling, completion, and treatment of subterranean formations penetrated by wellbores. For example, hydraulic fracturing is often practiced as a means to enhance recovery.
During hydraulic fracturing, a viscous well treatment fluid is injected into a well bore under high pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation that generally continues to grow during pumping. As the fracture widens to a suitable width during the course of the treatment, a proppant (e.g., sand grains, aluminium pellets, or other material), may then also be added to the fluid. The proppant remains in the produced fracture to prevent closure of the fracture and to form a conductive channel extending from the well bore into the formation being treated once the fracturing fluid is recovered. The treatment design generally requires the fluid to reach a maximum viscosity as it enters the fracture that affects the fracture length and width. The viscosity of most fracturing fluids is generated from watersoluble polysaccharides, such as galactomannans or derivatives thereof. Crosslinking agents, such as borate, titanate, or zirconium ions, are commonly added to increase the fluid viscosity.
Once a suitable amount of fractures are formed, it is generally desirable that the fluid viscosity decrease to levels approaching that of water after the proppant is placed. This allows a portion of the treating fluid to be recovered without producing excessive amounts of proppant after the well is opened and returned to production. The recovery of the fracturing fluid is accomplished by reducing the viscosity of the fluid to a lower value such that it flows naturally from the formation. Incorporating chemical agents, referred to as breakers or breaking agents, into the fluid can accomplish this viscosity reduction or conversion. Typically, these agents are either oxidants or enzymes that operate to degrade the polymeric gel structure.
In choosing a suitable breaker, one may consider the onset of viscosity reduction, i.e., breakage. Viscous well treatment fluids that break prematurely can cause suspended proppant material to settle out before being introduced a sufficient distance into the produced fracture. Moreover, premature breaking can result in a less than desirable fracture width in the formation causing excessive injection pressures and premature termination of the treatment.
On the other hand, viscous well treatment fluids that break too slowly can cause slow recovery of the fracturing fluid from the produced fracture, which delays hydrocarbon production. Still further, the proppant can dislodge from the fracture, resulting in at least partial closing and decreased efficiency of the fracturing operation. Preferably, the fracturing gel should begin to break when the pumping operations are concluded. For practical purposes, the gel preferably should be completely broken within about 24 hours after completion of the fracturing treatment.
In low-temperature wells, enzymatic breaking agents are often used, but they are relatively expensive in comparison to oxidizing breaking agents. In shallow wells, percarbonates are often used, but as the drilling gets deeper percarbonates provide premature breaking and are less preferred.
Accordingly, there is a need in the art for improved breaking agents that can be used in various settings, conditions, and oil well applications.